The present invention relates to sensors, and more particularly, to sensors which are actuated by a predetermined, minimum acceleration force, or rate of change of velocity.
While the sensing device of the present invention is equally adapted for use in any type of apparatus in which it is necessary to sense acceleration forces, it is especially useful for sensing a collision and arming or actuating a vehicle occupant restraint system and will be described in connection therewith. Therefore, it should be clearly understood that the term "acceleration" is intended to mean and include both the positive and negative, i.e., both acceleration and deceleration.
Among the acceleration sensors well known in the art, the one illustrated in U.S. Pat. No. 3,154,774 is typical of those which operate on the "inclined plane" principle. An inertial member, in this case a small cylindrical battery, is positioned at the bottom of an inclined plane so that the occurrence of the predetermined acceleration force causes the member to roll up the plane, and over the top, dropping down between a pair of open contacts which are closed by the inertial member.
Another class of sensors is represented by U.S. Pat. No. 3,688,063 which employs a rollerband device (see for example the rollerband device of U.S. Pat. No. 3,452,175) to effectuate the closing of the contacts when the predetermined acceleration force occurs.
Both of the types of devices described, as well as others known in the art, ultimately require the closure of a set of mechanical contacts, and because of the high g forces to which such devices are subject, acceleration sensors using mechanical contacts are inevitably troubled by contact bounce, poor contact and high contact resistance. In addition, many of the classes of prior art sensors, especially the inclined plane sensors, operate too slowly to be useful in devices such as passenger restraint systems, wherein the operation of the system components is measured in milliseconds.
Accordingly, it is an object of the present invention to provide an acceleration sensor which overcomes the problems associated with mechanical contacts, as discussed previously.
It is a more specific object of the present invention to provide an acceleration sensor adapted for use in a vehicle passenger restraint system which may be actuated within a matter of milliseconds, by a very precisely predetermined acceleration force, to provide an output signal which is highly reproducible.
In addition to the common problems and requirements for acceleration sensors as discussed above, when such sensors are used in occupant restraint systems, it is essential that they not be easily "short circuited" to prevent actuation of the firing circuit when no crash has occurred. It is also desirable that sensors intended for use in occupant restraint systems have a built-in diagnostic capability, so that any sensor malfunctions which may occur will not remain unnoticed.
Accordingly it is another object of the present invention to provide an acceleration sensor which is capable of indicating the continuing operability and readiness of each of the sensor components.
It is also an object of the present invention to provide an acceleration sensor which is less readily susceptible to being actuated by inadvertence or vandalism.